Dell Brocade DCX, Configuration Manual

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BigIron RX Series Configuration Guide 173
53-1001810-01
Configuring packet parameters
7
To configure the untagged max-frame-size on a VLAN, enter a command such as the following at he
Interface Configuration level.
BigIron RX (config-vlan-20)#
BigIron RX (config-vlan-20)#max-frame-size 5000
Please reload system!
BigIron RX (config-vlan-20)#
Syntax: max-frame-size <bytes>
The <frame-size> variable specifies the maximum frame size for each port that is connected the
same PPCR as described in
Table 47 . Values can be from 64 to 9212 bytes. The default is 1518
bytes.
Changing the MTU
The IP MTU is the maximum length of an IP packet that a Layer 2 packet can contain. If an IP
packet is larger than the IP MTU allowed by the Layer 2 packet, the device fragments the IP packet
into multiple parts that will fit into Layer 2 packets, and sends the parts of the fragmented IP
packet separately, in different Layer 2 packets. The device that receives the multiple fragments of
the IP packet reassembles the fragments into the original packet.
The default IP MTU is 1500 bytes for Ethernet II packets and 1492 for Ethernet SNAP packets. You
can change the IP MTU globally or an individual ports. You can increase the IP MTU size to
accommodate large packet sizes, such as jumbo packets, globally or on individual physical ports.
However, IP MTU cannot be set higher than the maximum frame size, minus 18.
For jumbo packet, the device supports hardware forwarding of Layer 3 jumbo packets. Layer 3 IP
unicast jumbo packets received on a port that supports the frame's IP MTU size and forwarded to
another port that also supports the frame's IP MTU size are forwarded in hardware.
Configuration considerations for Increasing the IP MTU
Consider the following before configuring the maximum value to increase the IP MTU:
•
The maximum value of an IP MTU cannot exceed the configured maximum frame size, minus
18. For example, global IP MTU cannot exceed the value of default-max-frame-size, minus 18
bytes. IP MTU for an interface cannot exceed the value of the maximum frame size configured
on a port, minus 18 bytes. The 18 bytes is used for IP overhead, VLAN tagging, etc.
•
When you increase the IP MTU size of a port, the increase uses system resources. Increase
the IP MTU size only on the ports that need it. For example, if you have one port connected to
a server that uses jumbo frames and two other ports connected to clients that can support the
jumbo frames, increase the IP MTU only on those three ports. Leave the IP MTU size on the
other ports at the default value (1500 bytes). Globally increase the IP MTU size only if needed.
•
Use the same IP MTU size on all ports that will be supporting jumbo frames. If the device
needs to fragment a jumbo frame (and the frame does not have the DF bit set), the device
fragments the frame into 1500-byte fragments, even if the outbound port has a larger IP MTU.
For example, if a port has an IP MTU setting of 8000 and receives an 8000-byte frame, then
must forward the frame onto a port with an IP MTU of 4000, the device does not fragment the
8000-byte frame into two 4000-byte frames. Instead, the device fragments the 8000-byte
frame into six fragments (five 1500-byte fragments and a final, smaller fragment.)